Spreadsheet Excel Writer - append data to same file?

Hey everyone. Thanks so much for all your help installing Pear and working around the various issues with me. I finally got it installed properly, manually installed Spreadsheet Writer and OLE, and have been generating spreadsheets using dynamic data. So, the formatting will take some time but seems to be pretty straightforward. The only question I have left is, how do I append data to the same .XLS file? In the site I'm working on, different users will visit the form page and submit information. Currently, I've got the Writer code opening the XLS, writing the lines of data to a row, and closing. But then it seems the next time I visit the page and enter data, it just over writes the old stuff. Any ideers? Thanks again Kevin